This invention relates to electroluminescent devices, and more specifically relates to thin film electroluminescent devices having a doped phosphor layer.
Thin film electroluminescent devices are well known, and are generally constructed from a series of thin films deposited onto a substrate. Typically, thin film electroluminescent devices consist of a transparent front electrode layer, a phosphor layer, and a back electrode layer. In an inorganic device, the phosphor layer is usually sandwiched between two dielectric layers. When an alternating voltage is applied across the electrodes, light is emitted from the phosphor layer. A detailed discussion of thin film electroluminescent devices can be found in, for example, U.S. Pat. No. 5,049,780, issued to DOBROWOLSKI et al., the contents of which are incorporated herein by reference. It is also known that the performance characteristics of the phosphor layer can be varied through doping of the phosphor material. Certain doped phosphor materials are discussed in Rack, P. D. and Holloway P. H., xe2x80x9cThe structure, device physics, and materials properties of thin film electroluminescent displaysxe2x80x9d, Materials Science and Engineering, R21, January 1998, the contents of which are incorporated herein by reference.
Doped phosphor emitter materials are also taught in WIPO publication WO98/21919, published May 22, 1998 to VELTHAUS. According to VELTHAUS, the phosphor is made out of host crystal material from a compound of one or more earth-alkaline metals (or zinc or cadmium), in which these metals are present in the form of calcogenide, for example as sulphide. The host crystal is doped with traces of a rare earth, for example cerium or bismuth, and with additional traces of silver. VELTHAUS further teaches that a host lattice of SrS:CeCl3 can be used to obtain blue/green light emission, and by adding Ag to the host lattice, a shift of emissions into the blue spectrum is achieved. As stated in VELTHAUS, this host lattice results in a highly crystalline phosphor layer. However, in certain applications, such high crystallinity is undesirable, as it can increase reflectivity, and/or decrease device life time. Further, VELTHAUS teaches phosphors that include a rare earth or bismuth, and therefore, is not well suited for use in ZnS:Mn based devices.
It will be apparent from the foregoing that prior art electroluminescent devices are generally designed with large-grain crystalline phosphor layers which can be undesirable in certain applications including certain high-contrast and/or high-reliability devices.
It is an object of the present invention to provide a novel thin film electroluminescent device which obviates or mitigates at least one of the above-described disadvantages of the prior art.
In a first aspect of the invention, there is provided an electroluminescent device comprising: a pair of electrodes of which at least one of the electrodes is transparent to electroluminescent light, and a phosphor layer disposed between the electrodes. The phosphor layer has a host crystal lattice, a first dopant and a second dopant where the first dopant cooperates with the host crystal lattice to cause light emission when a voltage is applied across the pair of electrodes, and the second dopant further distributes the first dopant in the host crystal lattice to increase light emission from the phosphor layer.
In one particular aspect of the first aspect of the invention, the electroluminescent device further comprises at least one dielectric layer disposed between the phosphor layer and at least one of the pair of electrodes. The dielectric layer is chosen from the group consisting of Al2O3, Y2O3, SiON, SiO2, Ta2O5, and BaTiO3.
In another particular aspect of the first aspect of the invention, the host crystal lattice is a wide band gap semiconductor. The wide band gap semiconductor is chosen from the group consisting of ZnS, ZnSe, ZnSSe, CaS, SrS, SrCaS and BaS.
In yet another aspect of the first aspect of the invention, the first dopant is chosen from the group consisting of Mn, Tb, Ho, Ce and Cu.
In a particular preferred aspect of the first aspect of the invention, the first dopant has a concentration of about 0.1% to about 2% by weight of the host crystal lattice. In a second preferred aspect, the first dopant has a concentration of about 0.2% to about 1% by weight of the host crystal lattice. It is particularly preferrred that the first dopant has a concentration of about 0.3% to about 0.8% by weight of the host crystal lattice. Typically, the first dopant has a preferred concentration of about 0.6% by weight of the host crystal lattice.
In keeping with another aspect of the first aspect of the invention, the second dopant is a Group IB metal. As is known to those of skill in the art, the Group IB metal includes Cu, Ag and Au. Preferably, the second dopant is Ag. In a particular aspect of the first aspect of the invention, the second dopant has a concentration of about 0.25% to about 2% of the first dopant concentration. More particularly, the second dopant has a concentration of about 0.5% to about 1.5% of the first dopant concentration. More particularly, the second dopant has a concentration of about 0.6% to about 1.2% of the first dopant concentration. Typically, the second dopant has a concentration of about 1% of the first dopant concentration.
In an alternative aspect, there is provided a phosphor material comprising: a host crystal lattice, an emitter material doped with the host crystal lattice, and a displacer material doped concurrently with the emitter material to the host crystal lattice for urging the emitter material into substitutional positions within the host crystal lattice.
In another alternative aspect, there is provided a phosphor material comprising: a host crystal lattice, an emitter material doped with the host crystal lattice, and a displacer material doped concurrently with the emitter material to the host crystal lattice for urging the emitter material into light emissive positions within the host crystal lattice such that energy interactions between the emitter material during excitation are reduced and light emissions therefrom are increased.
In yet another aspect of the invention, there is provided an electroluminescent device comprising: a pair of electrodes of which at least one of the electrodes is transparent to electroluminescent light; a phosphor layer disposed between the electrodes; at least one dielectric layer disposed between the phosphor layer and at least one of the pair of electrodes; and a substrate layer above which one of the pair of electrodes, the at least one dielectric layer, the phosphor layer, and the one other of the pair of electrodes are successively deposited. The phosphor layer has a host crystal lattice, a first dopant and a second dopant where the first dopant cooperates with the host crystal lattice to cause light emission when a voltage is applied across the pair of electrodes, and the second dopant further distributes the first dopant in the host crystal lattice to increase light emission from the phosphor layer. Typically, the dielectric layer is Al2O3, the host crystal lattice is ZnS, the first dopant is Mn, and the second dopant is Ag. It is presently preferred that the first dopant has a concentration of about 0.6% by weight of the host crystal lattice, and that the second dopant has a concentration of about 1% of a first dopant concentration.
In a second aspect of the invention, there is provided a method of assembling at least a portion of an electroluminescent device. The method comprises the steps of: depositing a first electrode above a substrate, forming a phosphor layer above the first electrode, in which the phosphor layer is formed by depositing a host crystal lattice, a light emitting dopant and a dispersing dopant substantially simultaneously above the first electrode such that the dispersing dopant urges at least a portion of the light emitting dopant into positions within the host crystal lattice that are favourable to light emission, and the step of depositing a second electrode above the phosphor layer.
In one particular aspect of the invention, the electroluminescent device further comprises at least one dielectric layer disposed between the phosphor layer and at least one of the pair of electrodes. Thus, the method of assembling such an electroluminescent device further comprises the step of depositing the at least one dielectric layer onto the at least one of the pair of electrodes and/or phosphor layer.
In one aspect of the invention, deposition of the host crystal lattice is controlled using a standard quartz crystal monitor. In another aspect of the invention, deposition of the first dopant and second dopant is controlled using Knudsen cells. In yet another aspect of the invention, deposition of the first dopant and second dopant is controlled using a chopper wheel. In an alternative aspect of the invention, formation of the phosphor layer is chosen from the group of deposition methods consisting of sputter, ebeam deposition, and atomic layer epitazy.
The host crystal lattice is typically ZnS, and the first dopant is Mn or the first dopant is Ho. Typically, the second dopant is Ag.
In a particular preferred aspect of the second aspect of the invention, the deposition temperature of the first dopant is from about 600xc2x0 C. to about 1200xc2x0 C. In another preferred aspect, the deposition temperature of the first dopant is from about 650xc2x0 C. to about 925xc2x0 C. Typically, the deposition temperature of Mn is about 915xc2x0 C. Furthermore, the deposition of Ho is typically from about 700xc2x0 C. to about 850xc2x0 C. In yet another preferred aspect, the deposition temperature of the second dopant is from about 600xc2x0 C. to about 1200xc2x0 C. It is preferred that the deposition temperature of the second dopant is from about 700xc2x0 C. to about 1100xc2x0 C. Typically, the deposition temperature of Ag is from about 700xc2x0 C. to about 730xc2x0 C.
In a third aspect of the invention, there is provided a method of assembling at least a portion of an electroluminescent device, comprising the steps of: depositing a first electrode above a substrate, depositing at least one dielectric layer onto the first electrode, forming a phosphor layer above the first electrode in which the phosphor layer is formed by depositing a host crystal lattice, a light emitting dopant at a temperature of about 650xc2x0 C. to about 925xc2x0 C., and a dispersing dopant at a temperature of about 600xc2x0 C. to about 1200xc2x0 C. substantially simultaneously above the first electrode such that the dispersing dopant urges at least a portion of the light emitting dopant into positions within the host crystal lattice that are favourable to light emission, and depositing a second electrode above the phosphor layer.